Grain boundaries of covalent bonding materials such as silicon, germanium and SiC require rather physical consideration than that of geometry to discuss the result of their electron microscopic observation. Dangling bond, for example, is expected least in number in the stable boundary (least dangling bond rule).  in practice, further precise consideration must be done both in geometry and in physics in the covalent bonding materials boundary. in the present study, geometrical discussion is combined together with that of the atomic bonding about diamond, SiC and ZnO grain boundaries. Chemical structure image of the modern high-voltage transmission electron microscope enabled to do it. An asymmetrical Σ3 boundary of diamond and a 6H-SiC boundary with 70.5 degree rotation are mainly observed.
The sliced specimen, 0.3mm in thickness, was mechanically thinned down to 0.02 mm and was finally polished using an argon ion milling. Final thickness of the specimen was estimated to be 3 nm in the region close to the edge of the small hole.